EXPLORING INTERMEDIATE (5–40 AU) SCALES AROUND AB AURIGAE WITH THE PALOMAR FIBER NULLER 1 1 1 1 1 2 1 J. K uhn¨ , B. Mennesson ,K.Liewer , S. Martin ,F.Loya , R. Millan-gabet , and E. Serabyn 1 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA; [email protected]fl.ch 2 NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA 91125, USA ABSTRACT We report on recent Ks-band interferometric observations of the young pre-main-sequence star AB Aurigae obtained with the Palomar Fiber Nuller (PFN). Reaching a contrast of a few 10−4 inside a field of view extending from 35 to 275 mas (5–40 AU at AB Aur’s distance), the PFN is able to explore angular scales that are intermediate between those accessed by coronagraphic imaging and long baseline interferometry. This intermediate region is of special interest given that many young stellar objects are believed to harbor extended halos at such angular scales. Using destructive interference (nulling) between two sub-apertures of the Palomar 200 inch telescope and rotating the telescope pupil, we measured a resolved circumstellar excess at all probed azimuth angles. The astrophysical null measured over the full rotation is fairly constant, with a mean value of 1.52%, and a slight additional azimuthal modulation of ±0.2%. The isotropic astrophysical null is indicative of circumstellar emission dominated by an azimuthally extended source, possibly a halo, or one or more rings of dust, accounting for several percent of the total Ks-band flux. The modest azimuthal variation may be explained by some skewness or anisotropy of the spatially extended source, e.g., an elliptical or spiral geometry, or clumping, but it could also be due to the presence of a point source located at a separation of ∼120 mas (17 AU) with ∼6 × 10−3 of the stellar flux. We combine our results with previous Infrared Optical Telescope Array observations of AB Aur at H band, and demonstrate that a dust ring located at ∼30 mas (4.3 AU) represents the best-fitting model to explain both sets of visibilities. We are also able to test a few previously hypothesized models of the incoherent component evident at longer interferometric baselines. Key words: instrumentation: interferometers – protoplanetary disks-stars: individual (AB Aurigae) – stars: pre-main sequence – stars: variables: T Tauri, Herbig Ae/Be – techniques: interferometric 1. INTRODUCTION homogeneous hot (2500–3500 K) gaseous contribution inside of it (Tannirkulam et al. 2008). These two inner components are Planet formation and migration processes can currently be found about equally bright, and together contribute 70% of the studied through the direct observation of planets with masses of total flux at H band (1.65 μm), and 85% at K band (2.2 μm). In tens of MJ at large distances (tens of AU) from young (<10 Myr) addition, closure phase (CP) H-band observations at the Infrared stars. At 144 pc from us, the young (4 ± 1 Myr) massive (M ∼ Optical Telescope Array (IOTA) have suggested an off-axis 2.4 ± 0.2 M; DeWarf et al. 2003) pre-main-sequence star partially resolved feature farther out, at 1–5 AU scales (Millan- AB Aurigae (AB Aur) presents as an ideal prototype. As a Gabet et al. 2006b). Finally, spectral visibilities obtained with member of the Herbig Ae class, AB Aur has been extensively the VEGA spectrometer at CHARA in R band have also investigated in the near-infrared (NIR), notably by direct and suggested the presence of an unknown component at about polarimetric imaging of the outer cold disk, which extends out 5.5 AU (38 mas) contributing about 7% of the flux (Rousselet- to hundreds of AU. In general, the AB Aur disk is thought to Perraut et al. 2010). In summary, there is evidence for emission be seen nearly face-on, seen with an inclination at or below outside a field of view (FOV) of 1 AU, at best only partially 30◦ (Eisner et al. 2003; Fukagawa et al. 2004). The cold outer resolved with previous interferometric observations. Indeed, gaseous nebula (Grady et al. 1999) has been reported to exhibit this intermediate radial regime, i.e., between the interferometric spiral geometry with several trailing spiral arms up to 450 AU and single aperture observational regimes, has hitherto not been away from the host star (Fukagawa et al. 2004). In the case of accessed observationally in the NIR. the outer cold dust envelope, it was detected out to 130–140 AU, Other non-CP interferometric results generally assumed some reportedly presenting both an azimuthal gap at a separation of extended uniform emission (i.e., a “halo”) on top of the reported ∼100 AU, as well as two rings at radial distances of ∼40 and best-fit models, accounting for 5%–10% of the NIR flux, and 100 AU, separated by an annular dip (Hashimoto et al. 2011; typically extending from 1.5 to 70 AU (10–500 mas). This ex- Oppenheimer et al. 2008; Perrin et al. 2009). There is also tended “halo” component was required to explain the visibility evidence of spiral-shaped asymmetry and warped geometry deficit systematically measured at short interferometric base- for the closer inner disk structure at a separation of ∼40 AU lines (∼10 m; Monnier et al. 2006; Tannirkulam et al. 2008). (Hashimoto et al. 2011). The spatial extension of such a putative halo was constrained On the other hand, interferometric investigations (10–350 m by speckle interferometry observations at K band (Leinert et al. baselines) that probe the inner 1 AU, have reported a similarly 2001) with visibilities near unity for spatial scales beyond 29 AU complex picture very close to the star. Most of the NIR (200 mas). In the mid-IR (10 μm), faint halo emission of about excess was mapped interferometrically and modeled as a ring 10% of the flux has, however, been reported at larger separation of hot dust (1800–1900 K) with a sharp inner rim located at beyond 0.5(Marinas˜ et al. 2006; Monnier et al. 2009), albeit 0.2–0.3 AU (Millan-Gabet et al. 1999, 2006b),withasmooth the bulk of the 10 μm emission was measured to be contained 1 0.012 1% point−source at 60 mas 0.010 1% point−source at 120 mas 0.008 0.006 0.004 Astrophysical Null 0.002 0.000 0 50 100 150 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 PFN baseline azimuth on−sky [degree E of N] Figure 1. Left: PFN transmission function (fringe pattern) on the sky at Palomar. The dashed circle represents a radius of λ/D for the 5.1 m Hale Telescope at Ks band, and the arrows the hypothetic trajectories of potential point-source companions relative to the fringes. The plotted image size is 600 × 600 mas (the map is computed up to 300 mas separation radius). Right: expected astrophysical null modulation that would be observed for these two point sources carrying 1% of the flux of the unresolved star. The azimuthal modulation frequency provides information on separation, while the position angle can be retrieved from the phase offset (modulo 180◦). within 35 mas (5 AU). Such “halo” features have been reported the presence of a companion within 2 AU of Vega (Mennesson around young stellar objects (YSOs; Leinert et al. 2001) and et al. 2011b). The PFN causes the Hale Telescope pupil to be FU Orionis objects (Millan-Gabet et al. 2006a), albeit probably re-imaged and then rotated by means of an actuated K-mirror, with different origins, but are still not well understood. which has the effect of rotating an equivalent fixed baseline of The AB Aur system presents a complex picture, probably b = 3.2 m between a pair of elliptical apertures of 3 × 1.5 m. including unseen ongoing planetary formation processes, at a This enables us to explore the close environment of a star and variety of scales. However, the gap of observational coverage in look for azimuthal variations. In the case of an unresolved naked between NIR interferometry and direct imaging lies at critical star, or of a uniform disk seen face-on, the measured astrophysi- planet formation scales, ranging from ∼1 to 40 AU. With the cal null will show no modulation with baseline rotation, at least Palomar Fiber Nuller (PFN; Martin et al. 2008; Mennesson down to the current measurement accuracy of a few 10−4.On et al. 2011a, 2010; Serabyn et al. 2010), we can now access the other hand, non-axisymmetric source structure, including most of this scale, i.e., the outer solar system scale past the presence and position—modulo 180◦ for the position angle ∼4 AU, in the NIR for the first time. Here we report on (P.A.) of an off-axis source—will be revealed by a modulation Ks-band nulling observations of AB Aur, obtained with the of the null signal as the source flux is modulated by the PFN PFN 3.2 m interferometric baseline, that cover a 180◦ range on-sky transmission (Figure 1). in azimuth. With an effective search space extending from AB Aur was observed with the PFN in Ks band on the night ∼35 mas (5 AU, inner half transmission point) to ∼275 mas of 2012 October 30 UT, at six different azimuth baselines (40 AU, outer half transmission point), the PFN is ideally suited spanning 180◦ on-sky in steps of 30◦, all obtained under to provide measurements at these missing scales.